It's possible those people used a bogus email address. That can happen if someone deletes their email account or uses a fake email to sign up, thinking they will access free content on the 'thank you' page.

You might as well delete these people as they are not receiving your emails anyway.

Hi - Getresponse does some automatic list cleaning for you for bounces - there are some details on this here What are Bounces and Complaints? - GetResponse Support

You can use list hygiene to remove email address's from your list. This allows you to clean the list down. For example you can remove subscribers that have never opened emails or open a very small amount of them. This helps keep the list size down and can save you money on the monthly subscription fees.

Getresponse allows you to do an advanced search that you can export to a CSV. You can then use this list in list hygiene and either unsubscribe or black list them

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Is a 10% or more bounce rate considered high? What would be considered normal?

If certain emails get bounced back every time then the addresses must be fake.Just get rid of em

You can remove bounce emails by verifying your lists... remove failed emails from your list that's it...

It depends whether is is soft bounce or hard bounce.

Soft bounces are existing email address with your email not delivered due to full inboxes, get response email servers busy, etc

Hard bounces are invalid email address and get response delete them automatically so you don't have to do anything.

Regarding email hygiene, I delete on regular basis subscribers who haven't open email from me in last 90 days.

The reason for that is two fold.

First, by deleting inactive subscribers from my list I actually increase my email statistics.
Second, I paid less for Get Response service.